(a) Field of the Invention
This invention relates to frictional forced power transmission belts for transmitting power with their belt body wound around and in contact with a pulley, and relates to belt drive systems with the same.
(b) Description of the Related Art
Widespread use is made of serpentine belt drive systems for transmitting power from a crank shaft of an automotive engine to accessories such as a power steering, an air compressor and an alternator via a single V-ribbed belt in order to reduce the space for an engine room. Under these circumstances, such a V-ribbed belt is required to have a high power transmission capacity. In addition, the V-ribbed belt is also required to have a high noiselessness during belt run for the pursuit of a comfortable ride on a vehicle and other purposes. For these purposes, a typical V-ribbed belt is reinforced by short fibers mixed in a compression rubber layer that makes contact with pulleys so as to be oriented in a belt widthwise direction, and the short fibers protrude beyond the belt surface to reduce the coefficient of friction of the belt surface, resulting in reduced noise production and improved abrasion resistance.
Japanese Unexamined Patent Publication No. 7-63241 discloses a power transmission V-belt comprising an adhesion rubber layer in which a cord is embedded along a belt lengthwise direction and a compression rubber layer which adjoins the bottom face of the adhesion rubber layer and in which short fibers are embedded along a belt widthwise direction so that some of the widthwise short fibers protrude beyond the belt side faces. In this V-belt, the short fibers in the compression rubber layer are formed of para-aramid and/or meta-aramid fibers of 2 mm to 10 mm length oriented in the belt widthwise direction, and pulp-like para-aramid fibers some of which have a largest length of 2 mm and 80% or more of which have a length of 1 mm or less. This publication describes that the power transmission V-belt sufficiently exhibits the effects of aramid fibers mixed in the compression rubber layer of the belt and concurrently improves the abrasion resistance and non-stickiness of the compression rubber layer.
Japanese Unexamined Patent Publication No. 2001-165244 discloses a V-ribbed belt in which a compression rubber layer contains short cotton fibers and short para-aramid fibers some of which protrude from the rib surface, the protruding short para-aramid fibers are fibrillated, and the contents of the short cotton fibers and the short para-aramid fibers are 10 to 40 parts by weight and 5 to 10 parts by weight, respectively, with respect to 100 parts by weight of rubber in the compression rubber layer. This publication describes that in use as an automotive belt for an engine having large variations in rpm, the V-ribbed belt can eliminate noise production by suppressing minute slips during wet run.
Japanese Unexamined Patent Publication No. 2002-227934 discloses a V-ribbed belt that has a structure in which a cord is embedded in an adhesion rubber layer along the belt lengthwise direction, a compression rubber layer with a plurality of ribs extending in the belt lengthwise direction is stacked on the bottom of the adhesion rubber layer, a cover fabric is stacked as a tension layer on the top of the adhesion rubber layer, and the compression rubber layer contains 5 to 40 parts by mass of short fibers with respect to 100 parts by mass of its rubber component. The publication describes that there can be obtained a power transmission belt having excellent power transmission capacities during both normal and wet runs and a high noise reduction effect at low tension.
Japanese Unexamined Patent Publication No. 2003-12871 discloses a V-ribbed belt formed of an elastic body layer including an adhesion rubber layer in which a cord is embedded along the belt lengthwise direction and a compression rubber layer, wherein out of the elastic body layer consisting of the adhesion rubber layer and the compression rubber layer, at least the compression rubber layer is made of a vulcanizate of an ethylene-α-olefin elastomer compound, the ethylene content of the ethylene-α-olefin elastomer is 60 to 75 mass %, one or more kinds of short fibers of 0.5 mm to 3 mm length are contained as reinforcing fibers in the compression rubber layer, and the total amount of addition of the short fibers is 10 to 30 parts by mass to 100 parts by mass of the ethylene-α-olefin elastomer. This publication describes that such a short fiber-containing rubber composition excellent in sheetability is used at least for the compression rubber layer, thereby providing a high-durability power transmission belt having excellent flexural fatigue resistance and heat resistance, cold resistance, abrasion resistance and sticking abrasion resistance.
Japanese Unexamined Patent Publication No. 6-288439 discloses a power transmission belt which is made of urethane elastomer but in which powders of polyolefin such as polyethylene or polypropylene are dispersed. This publication describes that there can be obtained a power transmission belt of urethane elastomer that has an excellent abrasion resistance, does not break even when driven at high load and high speed under high-temperature conditions, and thereby exhibits a long life.
When the amount of short fibers mixed into the compression rubber layer of a V-ribbed belt is increased, the number of short fibers exposed at the belt surface is also increased and therefore the effects of improving noise reduction performance and abrasion resistance becomes large.
Increasing the amount of short fibers mixed, however, makes it difficult to uniformly knead the rubber composition containing short fibers for the formation of a compression rubber layer and mould it in the shape of a sheet. For this reason, only at most 25 parts by mass of short fibers can be mixed into 100 parts by mass of a rubber component of the compression rubber layer. Furthermore, the short fibers are fabricated by cutting long fibers, and need to be subjected to adhesion treatment in order to be given adhesiveness to the rubber, resulting in high material unit cost. Therefore, an increase in the amount of short fibers mixed leads to a rise in the material cost of the belt.